A concentrated animal feeding operation, or CAFO, is near a canal whose water came in contact with the affected romaine lettuce from the Yuma, Arizona, growing region, the FDA said. The water may have been used for irrigation, and sample testing revealed in June that it was tainted with E. coli O157:H7 that had the same genetic fingerprint as the outbreak strain.

This was the largest multi-state E. coli outbreak in the U.S. in a dozen years. The contaminated lettuce sickened 210 people in 36 states and killed five. Ninety-six victims landed in the hospital, 27 of whom developed kidney failure.

"The CAFO can hold in excess of 100,000 head of cattle at any one time, and the FDA traceback information showed a clustering of romaine lettuce farms nearby," the federal government said Monday. "Our experts continue to work on examining potential links between the CAFO, adjacent water and geologic and other factors that may explain the contamination and its relationship to the outbreak."

The name and the operator of the CAFO the FDA is investigating hasn't been released.

Some CAFOs are criticized for the strong stench emanating from their farms, caused by the large amount of feces generated by their animals.

E. coli live in the environment, foods, and people's and animals' intestines and fecal matter, according to the Centers for Disease Control and Prevention. One often-cited piece of advice is to wash one's hands carefully after using the bathroom or having contact with animals or places where they live, like farms and petting zoos.

The bacteria in animal waste could make their way into water one of two ways, explained University of Florida food-safety expert Keith Schneider. Water can run downhill, especially after rain, and make it into an irrigation ditch, or water can seep into an underground aquifer.

He advocates testing water before it's used for irrigation or, in the case of bagged produce, washing the vegetables.